The present invention is generally related to devices which generate analogue output voltages based on digital input values.
Devices which generate an analogue output voltage based on a digital input value are well known as Digital to Analogue Converters (DAC).
Digital to Analogue Converters are for example used to provide control voltages for the alignment of oscillation frequencies of transceiver circuits in devices for wireless communications like mobile phones, cordless phones or short range radio interfaces. For transmitter circuits a control voltage is necessary to align the voltage of a voltage controlled oscillator. The modulation frequency for a signal which has to be transmitted can be aligned this way. Also if the demodulation frequency of a receiver circuit has to be aligned Digital to Analogue converters are used.
Known Digital to Analogue converters can be constructed by weighted currents. The conventional circuits are based on analogue technology which is preferably designed for usage in analogue systems. This kind of implementation of Digital to Analogue Converters has the disadvantage that they need a large area of an integrated circuit.
Furthermore the usage of an analogue technology for the conventional Digital to Analogue Converters is not desirable if other parts of an electric device, where the digital to analogue conversion is needed for, are already designed in digital technology.
In the prior art Digital to Analogue Converters with digital technology are known. They are based on the so called xe2x80x9cPulse Width Modulation (PWM)xe2x80x9d. The analogue voltage will be generated in the way that a binary pulse signal is generated in dependence on a digital input value which is compared with a value from a counter. This binary pulse signal is low pass filtered with a conventional low pass filter, for example a RC-filter.
The approach has the disadvantage that it generates disturbing frequencies which requires complex filter structures to obtain a smooth output voltage.
Because of the complex filtering the desired output voltage will be reached slowly. The DAC based on Pulse Width Modulation cannot provide a new desired analogue output voltage based on a new digital input value fast.
According to the invention there is provided a device for converting a digital input into an analogue output comprising means for generating a binary pulse signal in dependence on said digital input, a filter for filtering said binary pulse signal to obtain said analogue output, where the level of the analogue output depends on the relation of high and low level pulses of the binary pulse signal during a period and the precision of the analogue output depends on the number of binary pulses during the period, characterised in that the high and low level pulses in the binary pulse signal are spread through said period. The analogue output might be a voltage.
The usage of Digital to Analogue Converters based on Pulse Width Modulation has shown that complex RC-filter structures are necessary to obtain a smooth analogue output voltage. The filtering is necessary to remove disturbing frequency components from the analogue output voltage because the generated binary pulse signal contains long sequences with high or low pulses, this leads to the generation of disturbing frequency components at low frequencies.
For the invention the binary pulse signal is generated in a way that the pulses are spread through the period and any disturbing frequency components on the analogue output voltage at higher frequency regions and the power of individual disturbing frequencies is lower. Now with the present invention a less complex RC-filter can be designed.
The filtering can be made even more simple if the high and low level pulses in the binary pulse signal are spread randomly through said period. The method preferably will be conducted with a binary pulse signal which starts with a start value and restarts after all possible bit combinations within the bit sequence have been generated. This means that the binary pulse signal is repeated periodically. The bit sequence can be a Pseudo Random Binary Sequence (PRBS). PRBS sequences can be produced by a shift register with feedback loops.
PRBS sequences can be selected among an indefinite number of PRBS sequences. Today they are used for the synchronisation and encryption in radio communication systems or to simulate normal traffic in digital transmission systems.
It is possible to produce only one binary pulse signal within one device and compare this binary sequence with several digital input values. In this way several output voltages can be obtained although the binary pulse signal is produced only once.
In a further advantageous embodiment of the invention the first pulses of the binary pulse signal are high level pulses or low level pulses until the desired analogue output voltage, which is defined by the digital input value, is reached. The number of necessary pulses to reach the desired output voltage may be given by a delay counter and a polarity register may give the information if high or low level bits are necessary to reach the desired output voltage.
It is also possible to generate said binary sequence with a digital filter that models the analogue filter. Based on the digital input value and the adjustments of the digital filter a binary pulse signal is generated where the relation of high and low level bits during a period to obtain the desired analogue output voltage after filtering said binary pulse signal. The digital filter contains a feedback line to compare the output signal of the digital filter with the digital input value.
The digital filter can furthermore be constructed in a way that said digital filter switches from a transient phase to a stationary phase when the result of the comparison between the output of said digital filter and the digital input is the first time zero. A transient phase is necessary if the output voltage is to be reached as quickly as possible. During the transient phase the coefficients of the digital filter are defined in a way that the binary pulse signal contains a higher number of high level bits if a higher output voltage has to be reached quickly.
The coefficients of the digital filter are defined in a way to generate a binary pulse signal which holds said analogue output voltage on the desired constant level according to the digital input during the stationary phase.
Inventive Digital to Analogue converters as defined here can be used to align transceiver circuits in mobile phones, cordless phone systems, short range radio interfaces or other communication devices. For example to provide control voltages for the alignment of the oscillation frequencies of transceiver circuits. For transmitter circuits an analogue control voltage is necessary to align the voltage of a voltage controlled oscillator. The modulation frequency for a signal which has to be transmitted can be aligned this way. The DAC can also be used for the alignment of the demodulation frequency of a receiver circuit.